1. Field of the Invention
The present invention relates to an alkali etching liquid which exhibits a Ni contamination reduction effect and a Cu contamination prevention effect even during the alkali etching of silicon wafers of low resistivity, and also relates to an etching method using such an etching liquid.
2. Description of Related Art
In a typical silicon wafer production, as shown in FIG. 5, a silicon single crystal ingot 1 that has been drawn using an apparatus for pulling single crystal is cut into blocks of a specified size, further grinding of the outer diameter is performed, and the resulting ingot block is then subjected to a water cutting (slicing) step 2, thereby forming thin, disc-like wafers. An internal diameter saw, band saw, or wire saw or the like is used in this slicing step. Subsequently, each thin, disc-like wafer is washed, and the outer peripheral edges of the wafer are subjected to a beveling step 3, thereby preventing particles, which are caused by cracking or chipping at the edges of the wafer, and also preventing the crown phenomenon, wherein abnormal growth can occur at the peripheral portions of the wafer during epitaxial growth on the wafer. The wafer is then subjected to a mechanical lapping step 4 to smooth the wafer surface, thereby grinding down the irregularities on the wafer surface generated by the slicing step 2, and improving the surface smoothness and wafer parallelism. By subsequently conducting a wafer etching step 5, affected layers of the various process materials formed on the wafer surface during the mechanical treatment processes such as the block cutting, outer diameter grinding, slicing and lapping steps can be removed. Subsequently, the etched wafer surface is polished in a wafer surface polishing step 6, and then washed in a washing step 7 to remove any polishing agents or contaminants that have adhered to the polished wafer surface, thereby yielding a silicon wafer 8 with a high degree of smoothness. Depending on the targeted application, a portion of these steps may be either replaced or repeated a plurality of times, and other steps such as heat treatment and grinding steps may be added or substituted to achieve the desired process.
A silicon wafer that has passed through mechanical treatment processes such as block cutting, outer diameter grinding, slicing and lapping contains a damage layer, also known as an affected layer. This affected layer can cause a variety of problems, including inducing crystal defects such as slip dislocation in subsequent device production processes, lowering the mechanical strength of the wafer, and causing undesirable effects on the electrical properties of the wafer, and must consequently be completely removed. An etching treatment is normally used for removing this affected layer. Suitable etching treatments include acid etching using an acid etching liquid of mixed acid or the like, and alkali etching using an alkali etching liquid of NaOH or the like. Acid etching has a faster etching rate, meaning uniform etching of the wafer surface is more difficult, and as a result, it tends to be prone to a deterioration in the degree of smoothness of the lapped wafer. Accordingly, alkali etching, which enables more uniform etching and does not impair the degree of smoothness of the wafer, is more widely used.
However, the electronic industrial grade alkali solutions used in this type of alkali etching contains between several dozen ppb and several ppm of metal impurities. Examples of these metal impurities contained within the alkali solutions include nickel, chrome, iron, and copper, and of these, nickel, chrome and iron from the stainless steel materials used in the production of the alkali solutions exist in the highest concentrations. If an alkali solution containing these types of metal impurities is used, as is, as the alkali treatment liquid within a wafer etching step, then during etching, metal ions from the metal impurities may adhere to the wafer surface or diffuse into the wafer interior, causing a deterioration in the wafer quality, which can cause a marked deterioration in the properties of the semiconductor device formed using the wafer. One possible solution for preventing deterioration in the wafer quality caused by the alkali etching liquid is to use a high purity alkali solution. However, commercially available high purity alkali solutions are limited to the extremely expensive analytical grade alkali solutions, and the use of such solutions for industrial purposes is completely impractical from a cost perspective.
Examples of methods that have been disclosed for resolving the above problems include alkali solution purification methods, in which metallic silicon or a silicon compound is dissolved in the alkali solution, with the resulting reaction product then effecting a deionization of the metal ions within the alkali solution, or alternatively, in which hydrogen gas is dissolved in the alkali solution, thereby effecting a deionization of the metal ions within the alkali solution, as well as semiconductor wafer etching methods in which an alkali solution purification treatment is conducted by deionizing the metal ions within the alkali solution, and the resulting purified alkali solution is then used for etching a silicon wafer (for example, see patent reference 1). Furthermore, an alkali solution purification method in which a reducing agent with a more basic oxidation potential than the reversible potential of the metal ions within the alkali solution is dissolved in the alkali solution, thereby effecting a deionization of the metal ions within the alkali solution, as well as a semiconductor wafer etching method in which a metal ion deionization treatment is conducted by dissolving a reducing agent with a more basic oxidation potential than the reversible potential of the metal ions within the alkali solution in the alkali solution, and the resulting deionized alkali solution is then used for etching a silicon wafer have also been disclosed (for example, see patent reference 2). By using the methods of the patent references 1 and 2, the metal ions within an alkali solution can be significantly reduced and low cost using a simple operation. Furthermore, by conducting etching using an alkali solution with a significantly lowered metal ion concentration, the degree of metal contamination of the silicon wafer caused by the etching process can be dramatically reduced, enabling both deterioration in the wafer quality and deterioration in the properties of the semiconductor device to be effectively suppressed.
In addition, an etching liquid for a semiconductor material that is prepared by immersing stainless steel in an aqueous alkali solution for at least 10 hours has also been disclosed (for example, see patent reference 3). The etching liquid disclosed in the patent reference 3 is able to effectively prevent metal contamination of semiconductor silicon wafers.
Patent Reference 1: Japanese Unexamined Patent Application, First Publication No. Hei 09-129624 (claim 1, claim 6, claim 9, and paragraph [0049])
Patent Reference 2: Japanese Unexamined Patent Application, First Publication No. Hei 10-310883 (claim 1, claim 5, and paragraph [0039])
Patent Reference 3: Japanese Unexamined Patent Application, First Publication No. 2001-250807 (claim 1, and paragraph [0033])
However, even using the etching methods and etching liquids disclosed in the above patent references 1 to 3, the reduction in the quantity of metal contaminants that diffuse into the interior of the wafer during etching is still not entirely satisfactory, and further improvements are desirable. Reductions are particularly sought in the level of Ni contamination and Cu contamination that diffuses into the interior of a silicon wafer with a resistivity of no more than 1 Ω·cm following alkali etching.
An object of the present invention is to provide an alkali etching liquid for a silicon wafer, and an etching method that uses such an etching liquid, which enable Ni contamination of the silicon wafer caused by the alkali etching to be reduced at low cost.
Another object of the present invention is to provide an alkali etching liquid for a silicon wafer, and an etching method that uses such an etching liquid, which enable Cu contamination of the silicon wafer caused by the alkali etching to be prevented, and also enable a further reduction in the Ni contamination.